Hydrocyclone with turbulence creating means

ABSTRACT

In a hydrocyclone the separation chamber (2) has a circumferential wall (3) provided with at least one turbulence creating member (12), which extends along the circumferential wall and crosses a helical path (10) along which a liquid stream is generated during operation. According to the invention, the turbulence creating member is formed by an offset (12) on the circumferential wall (3). The offset is formed and dimensioned such that said liquid stream substantially loses its contact with the circumferential wall as the liquid stream passes the offset. As a result turbulence is created in a layer of the liquid stream situated closest to the circumferential wall, without the liquid stream developing any substantial flow component directed inward in the separation chamber.

The present invention relates to a hydrocyclone for separating a liquidmixture into a heavy fraction and a light fraction, comprising a housingforming an elongated separation chamber with a circumferential wall andtwo opposed ends, an inlet member for supplying the liquid mixturetangentially into the separation chamber at one end of the latter, anoutlet member for discharging separated heavy fraction from theseparation chamber at the other end of the latter, and an outlet memberfor discharging separated light fraction from the separation chamber.The hydrocyclone further comprises means for supplying the liquidmixture to the separation chamber via the inlet member, so that duringoperation a liquid stream is generated along a helical path having acentral axis in the separation chamber, said helical path extending fromthe inlet member to said outlet member for heavy fraction, and at leastone turbulence creating member extending in the separation chamber alongthe circumferential wall and crossing said path.

In a known hydrocyclone of this kind according to U.S. Pat. No.4,153,558 there are four turbulence creating members in the form ofaxial ridges on the circumferential wall. When such a ridge is passed bya liquid stream turbulence is created in a layer of the liquid streamlocated closest to the circumferential wall, which prevents growth ofdeposits on the circumferential wall. Unless growth of the deposits isprevented during operation, the deposits might finally clog the outletmember for heavy fraction.

However, the liquid stream will become an inwardly directed component ofmovement into the separation chamber when the liquid stream passes eachridge, which means that separated light fraction will contain a largeamount of heavy components which were supposed to be discharged withseparated heavy fraction. This is particularly a drawback whenseparating liquid mixtures constituted by fibre suspensions, which willbe explained more closely in the following.

In the pulp and paper industry hydrocyclones are frequently used forcleaning fibre suspensions from undesired heavy particles. Thus, thefibre suspensions are separated into heavy fractions containing saidundesired heavy particles and light fractions containing fibres. Atypical hydrocyclone plant for this purpose has hydrocyclones arrangedin several stages of hydrocyclones coupled in parallel (normally threeor four stages), the hydrocyclone stages being coupled in series witheach other. Separated heavy fraction from the first hydrocyclone stageis once more separated in the second hydrocyclone stage, since saidheavy fraction also contains fibres, whereafter separated heavy fractionfrom the second hydrocyclone stage is separated in the thirdhydrocyclone stage, and so on. In this manner fibres are recovered stepby step from created heavy fraction. Light fraction, containingrecovered fibres formed in a hydrocyclone stage, is supplied back to thepreceding hydrocyclone stage. In this connection it is important thatthe hydrocyclones, at least in the first hydrocyclone stage, separateefficiently, so that the light fraction contains as few heavy undesiredparticles as possible.

A problem in connection with separating a fibre suspension by means of ahydrocyclone is that tight mats of fibres can be developed on thecircumferential wall of the separation chamber. Heavy undesiredparticles are easily caught in such mats of fibres, which can result inclogging of the outlet member for heavy fraction. This problem iseliminated by the prior art kind of hydrocyclone described above,whereby the creation of tight mats of fibres on the circumferential wallof the separation chamber is counteracted by said ridges. However, adrawback to the prior art hydrocyclone is that during operation eachridge gives the flowing fibre suspension an inwardly directed componentof movement in the separation chamber, whereby an increased share of theundesired heavy particles follows separated light fraction containingfibres.

The object of the present invention is to provide a new improvedhydrocyclone of the prior art kind, which is capable of separating aliquid mixture such that created light fraction will be substantiallyfree from heavy components.

This object is obtained by means of a hydrocyclone of the kind describedinitially, which mainly is characterized in that immediately upstreamthe turbulence creating member in the separation chamber thecircumferential wall has a smooth surface along a first zone of thecircumferential wall, which is situated at a substantially constantdistance from said centre axis along at least a one fifth of thecircumference of the separation chamber; that the turbulence creatingmember is formed by an offset on the circumferential wall, which offsetextends from said first zone of the circumferential wall to a secondzone of the circumferential wall situated at a greater distance from thecentre axis than the first zone, the second zone extending forwards fromthe offset, as seen in the flow direction of said liquid stream; andthat the offset is formed and dimensioned such that during operationsaid liquid stream substantially looses its contact with thecircumferential wall, as the liquid stream passes the set-off. Hereby,turbulence is created in a layer of the liquid stream situated closestto the circumferential wall, without the liquid stream developing anysubstantial flow component directed against said centre axis.

The term "offset" is used here to mean a ledge formed on the face of awall by diminution of the wall thickness above.

When separating fibre suspensions by means of the new hydrocyclone alight fibre fraction thus is created containing substantially fewerundesired heavy particles as compared to the light fraction created at acorresponding separation by means of the above mentioned prior arthydrocyclone. In addition, it has surprisingly been proved that theheavy fraction created by means of the new hydrocyclone containssubstantially fewer fibres than the heavy fraction created by means ofthe prior art hydrocyclone. This surprising effect probably depends onthat the under pressure generated closest to the circumferential wall ofthe separation chamber, when the liquid stream passes the offset, causesthe flocks of fibres close to the circumferantial wall to expand, sothat the fibres in said fibre flocks are released from each other. Thereleased fibres having a relatively large specific surface separateeasier in direction inwards in the separation chamber than said fibreflocks having a relatively small specific surface.

Thus, the new hydrocyclone is capable of separating fibre suspensions,such that the created heavy fraction will be relatively thin. For thepulp and the paper industry the use of the new hydrocyclone means theadvantage that fewer hydrocyclones than previously are needed forcleaning fibre suspensions from undesired heavy particles, since createdheavy fraction from a hydrocyclone stage need not be diluted so muchbefore it is supplied to the next hydrocyclone stage.

Practical tests have proved that said first zone of the circumferentialwall of the separation chamber should be at least one fifth of thecircumference of the separation chamber, which means that at most fouroffset can be arranged equally divided around the circumference of theseparation chamber. However, an optimum turbulence creating effect isalready achieved with one or at most two offsets.

Said second zone extends suitably along at least a fifth part of thecircumference of the separation chamber, the distance between the secondzone and the centre axis decreasing along the circumference of theseparation chamber in direction away from the offset, as seen in theflow direction of said liquid stream. At the end of the second zone, thesecond zone has suitably substantially the same distance to the centreaxis as the first zone.

Preferably, the circumferential wall has a sharp edge where the firstzone borders to the offset, in order to facilitate that said liquidstream will loose its contact with the circumferential wall, as itpasses the offset.

According to a preferred embodiment of the new hydrocyclone theseparation chamber in a way known per se (see U.S. Pat. No. 4,156,485)is formed by a plurality, axially consecutively arranged cylindricalchamber portions, which are formed such that the cross-sectional area ofthe separation chamber decreases step by step towards the outlet memberfor heavy fraction, the chamber portions coinciding to form a straightline extending in parallel with the chamber portions. The advantage of aseparation chamber formed in this manner as compared to an ordinaryconical separation chamber is that the circumferential walls of thecylindrical chamber portions will not give rise to forces on separatedheavy particles directed against the axial flow direction of the liquidmixture. Therefore, separated heavy particles are prevented fromrotating along the circumferential wall of the separation chamberwithout an axial movement relative to the separation chamber and fromcausing local wear of the circumferential wall. Instead, heavy particlesare entrained by the liquid mixture to shelves extending between thechamber portions in the circumferential direction of the separationchamber. Via breaks formed in said shelves the heavy particles areentrained by the liquid mixture axially further in the separationchamber towards the outlet member for heavy fraction.

Preferably, said offset is situated in front of said straight linetouching the cylindrical chamber portions. The chamber portions aresuitably formed such that the one of two adjacent chamber portions whichis located next to the outlet member for heavy fraction has a transverseextension from said straight line to the offset which amounts to thecorresponding transverse extension of the other chamber portion reducedby at most the transverse extension of the offset. As a result theseparation chamber can be formed such that the shelves are provided withan additional break at the offset, which has the advantage thatseparated heavy particles are entrained by the liquid stream axially inthe separation chamber also at the area of each offset.

The invention is explained more closely in the following with referenceto the accompanying drawing, in which

FIG. 1 shows a hydrocyclone according to the invention,

FIG. 2 shows a section along the line II--II in FIG. 1,

FIG. 3 shows a cross-section through an alternative embodiment of thehydrocyclone according to FIG. 1,

FIG. 4 shows a preferred embodiment of the hydrocyclone according to theinvention, and

FIG. 5 shows a part view of a section along the line V--V in FIG. 4.

The hydrocyclone shown in FIG. 1 comprises a housing 1, which forms anelongated separation chamber 2 with a circumferential wall 3 and twoopposite ends. At one end the separation chamber 2 has an inlet part 4,which has a constant cross-sectional area along the axial extension ofthe separation chamber 2. The inlet part 4 of the separation chamberpasses into a conical part 5, which has a decreasing cross-sectionalarea in direction towards the other end of the separation chamber.

An inlet member 6 is arranged at the inlet part 4 for feeding a liquidmixture to be separated tangentially into the separation chamber 2. Atone end of the separation chamber 2 the housing 1 is formed with atubular outlet member 7 situated centrally in the inlet part 4 fordischarging separated light fraction from the separation chamber 2. Atthe other end of the separation chamber 2 the housing 1 is formed withan outlet member 8 for discharging separated heavy fraction from theseparation chamber 2. A pump 9 is adapted to pump the liquid mixture tothe separation chamber 2 via the inlet member 6, so that duringoperation a liquid stream is generated along a helical path 10 having acentral axis 11 in the separation chamber 2 from the inlet member 6 tothe outlet member 8 for heavy fraction.

The circumferential wall 3 has a smooth surface in a first zone A, whichis at a substantially constant distance from the central axis 11 alonghalf the circumference of the separation chamber 2. An offset 12 on thecircumferential wall 3 extends axially along the entire separationchamber 2 with a constant transversal extension. (As seen in across-section through the separation chamber 2 the transverse extensionof the offset 12 should not be less than 1% or more than 40% of thedistance between the circumferential wall 3 and the central axis 11).Along the circumference of the separation chamber 2 the offset 12extends from the zone I at the end of the latter, as seen in the flowdirection of said liquid stream, to a second zone B of thecircumferential wall 3 situated at a greater distance from the centreaxis 11 than the first zone A.

The second zone B has a smooth surface and extends forwards in the flowdirection from the offset 12 to the first zone A, the distance betweenthe second zone B and the central axis 11 decreasing successively alongthe circumference of the separation chamber 2 in direction from theoffset 12. At the end of the second zone B, as seen in the flowdirection, the zone B has the same distance to the centre axis as thefirst zone A.

The circumferential wall 3 has a sharp edge 13 where the first zone Aborders on the offset 12. As seen in a cross-section through theseparation chamber 2 the set-off 12 is curved from the edge 13 forwardsrelative to the flow direction of the liquid stream and outwardsrelative to the separation chamber 2 to the second zone B of thecircumferential wall 3. The offset 12 is connected to the second zone Bof the circumferential wall 3 such that no edge is formed on thecircumferential wall 3.

During operation of the hydrocyclone according to FIGS. 1 and 2 theliquid mixture to be separated is pumped by means of the pump 9tangentially into the separation chamber 2 via the inlet member 6, sothat a liquid stream is generated along the helical path 10 about thecentral axis 11. As the liquid stream passes the offset 12 it looses itscontact with the circumferential wall 3, whereby a local underpressureis created behind the offset 12 as seen in the flow direction. Saidunderpressure gives rise to turbulence in a layer of the liquid streamlocated closest to the circumferential wall, which prevents growth ofdeposits on the circumferential wall 3. Created heavy fraction of theliquid mixture is emptied from the separation chamber 2 via the outletmember 8, while created light fraction of the liquid mixture is emptiedfrom the separation chamber via the outlet member 7.

In FIG. 3 there is shown an alternative embodiment of the hydrocycloneaccording to the invention, in which the circumferential wall of theseparation chamber is provided with two opposed offsets 12 and 15. Inthis case the circumferential wall has a smooth surface along a zone Cimmediately upstream each offset, which zone C is situated at asubstantially constant distance from a central axis 16 in the separationchamber along a fourth part of the circumference of the separationchamber.

The hydrocyclone shown in FIGS. 4 and 5 comprises a housing 17, aseparation chamber 18, a circumferential wall 19, an inlet member 20, anoutlet member 21 for light fraction, and an outlet member 22 for heavyfraction, which have the same function as corresponding components inthe above-described hydrocyclone according to FIG. 1. The separationchamber 18 is formed by a plurality, axially consecutively arrangedcylindrical chamber portions 23 having various cross-sectional areas,the cross-sectional area of the separation chamber 18 being decreasedstep by step towards the outlet member 22. Between adjacent chamberportions 23 there are formed shelves 24 extending in the circumferentialdirection of the separation chamber 18. The chamber portions 23 areoriented such that their walls coincide to form a straight line 25extending from the top to the bottom of the hydrocyclone. The line ofcoincidence, 25, provides a break in the shells 24. In contrast to aconical circumferential wall the circumferential wall in the cylindricalchamber portion 23 will not give rise to forces on separated heavyparticles directed away from the outlet member 22 for heavy fraction.

An offset 26 on the circumferential wall 19 extends axially along theentire separation chamber 18 with a constant transverse extension and issituated in front of the straight line 25 which touches the chamberportions 23. Each chamber portion 23 has a cross-sectional area which inprinciple corresponds with the cross-sectional area of the separationchamber 2 shown in FIG. 2. The chamber portions 23 are designed suchthat the one of two adjacent chamber portions 23a and 23b which is nextto the outlet member 22 has a transverse extension from the straightline 25 to the offset, which is equal to the corresponding transverseextension of the other chamber portion 23a reduced by the transverseextension of the offset 26. As a result breaks are also formed in theshelves 24 at the offset 26. In FIG. 4 two adjacent shelves aredesignated with 24a and 24b, respectively, which also are shown in FIG.5.

During operation of the hydrocyclone according to FIGS. 4 and 5separated heavy particles will be entrained to the shelves 24 and leavethese via said breaks at the straight line 25, which touches the chamberportions 23, and via said breaks at the setoff 26. In other respects thefunction of the hydrocyclone according to FIG. 4 is analogous to theabove-described hydrocyclone according to FIG. 1.

I claim:
 1. A hydrocyclone for separating a liquid mixture into a heavyfraction and a light fraction, comprising a housing forming an elongatedseparation chamber with a circumferential wall and two opposed ends, aninlet member for supplying a liquid mixture tangentially into theseparation chamber at one end of the separation chamber, a first outletmember for discharging separated heavy fraction from the separationchamber at the other end of the separation chamber, a second outletmember for discharging separated light fraction from the separationchamber, means for supplying the liquid mixture to the separationchamber via the inlet member, so that during operation a liquid streamis generated along a helical flow path having a central axis in theseparation chamber, said helical path extending from the inlet member tosaid first outlet member for heavy fraction, and at least one turbulencecreating member, which extends in the separation chamber along thecircumferential wall and crosses said helical path,wherein immediatelyupstream of the turbulence creating member in said flow path in theseparation chamber the circumferential wall has a smooth surfaceextending along a first zone of the circumferential wall which issituated at a substantially constant distance from said central axis forat least a fifth of the circumference of the separation chamber, whereinthe turbulence creating member is formed by an offset on thecircumferential wall which offset extends from said first zone of thecircumferential wall to a second zone of the circumferential wallsituated at a greater distance from the central axis of the helical flowpath than the first zone, the second zone extending forward from theoffset in the flow direction of said helical flow path, the offset beingformed and dimensioned so that during operation said liquid streamsubstantially loses its contact with the circumferential wall, as theliquid stream passes the offset, whereby turbulence is created in alayer of the liquid stream situated closest to the circumferential wall,without the liquid stream forming any substantial flow componentdirected toward said central axis.
 2. A hydrocyclone according to claim1, wherein said second zone extends for at least one fifth of thecircumference of the separation chamber, the distance between the secondzone and the central axis of the helical flow path decreasing along thecircumference of the separation chamber in direction away from theoffset in the flow direction of said helical flow path.
 3. Ahydrocyclone according to claim 1, wherein at the end of said secondzone, in the flow direction of said helical flow path, the second zoneis at substantially the same distance from the central axis as saidfirst zone.
 4. A hydrocyclone according to claim 1, wherein thecircumferential wall has a sharp edge where said first zone of thecircumferential wall borders the offset.
 5. A hydrocyclone according toclaim 1, wherein in a cross-section through the separation chamber theoffset has a transverse dimension of between 1 to 40% of the distancebetween the circumferential wall and the central axis of the helicalflow path.
 6. A hydrocyclone according to claim 1, wherein the off-setextends transversely a constant distance axially along the separationchamber.
 7. A hydrocyclone according to claim 6, in which the separationchamber is formed by a plurality of adjoining axially arrangedcylindrical chamber portions, which are formed such that thecross-sectional area of the separation chamber decreases step by steptowards said outlet member for heavy fraction, the chamber portionsbeing aligned to define a straight line extending in parallel with thechamber portions, in each chamber portion said off-set being situated infront of said straight line.
 8. A hydrocyclone according to claim 7,wherein of two adjacent chamber portions the chamber portion nearest tosaid outlet member for heavy fraction has a transverse extension fromsaid straight line to the offset which amounts to the correspondingtransverse extension of the second chamber portion reduced by at mostthe transverse extension of the offset.